Developer Guide

FPGA Optimization Guide for Intel® oneAPI Toolkits

ID 767853
Date 12/16/2022
Public

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Document Table of Contents

Specify a Workgroup Size

Specify a maximum or the required workgroup size whenever possible. The Intel® oneAPI DPC++/C++ Compiler relies on this specification to optimize hardware use of the SYCL* kernel without involving excess logic.

  • If you do not specify the [[intel::max_work_group_size(Z, Y, X)]] or [[sycl::reqd_work_group_size(Z, Y, X)]] attribute in your kernel, the workgroup size assumes a default value depending on compilation time and runtime constraints.
  • If your kernel contains a barrier, the Intel® oneAPI DPC++/C++ Compiler sets a default maximum scalarized work-group size of 128 work-items.
  • If your kernel does not query any SYCL intrinsic that allow different threads to behave differently (that is, local or global thread IDs, or work-group ID), the Intel® oneAPI DPC++/C++ Compiler infers a single-threaded execution mode and sets the maximum work-group size to (1, 1, 1). In this case, the SYCL runtime also enforces a global enqueue size of (1, 1, 1), and loop pipelining optimizations are enabled within the Intel® oneAPI DPC++/C++ Compiler.
Deprecation Notice:

The [[cl::reqd_work_group_size(Z, Y, X)]] attribute is deprecated. Use the [[sycl::reqd_work_group_size(Z, Y, X)]] attribute.

To specify the work-group size, modify your kernel code in the following manner:

  • To specify the maximum number of work-items that the compiler provisions for a work-group in a kernel, insert the [[intel::max_work_group_size(Z, Y, X)]] attribute in your kernel source code.

    For example:

    constexpr unsigned MAX_WG_SIZE = 4;
    ...
    cgh.parallel_for<class kernelCompute>(
      nd_range<1>(range<1>(N), range<1>(wg_size)),
      [=] (nd_item<id> it)
      [[intel::max_work_group_size(1, 1, MAX_WG_SIZE)]] {
        auto gid = it.get_global_id(0);
        accessorRes[gid] = accessorIdx[gid] * 2;
    });

  • To specify the required number of work-items that the Intel® oneAPI DPC++/C++ Compiler provisions for a work-group in a kernel, insert the [[sycl::reqd_work_group_size(Z, Y, X)]] attribute in your kernel source code.

    For example:

    constexpr unsigned REQD_WG_SIZE = 4;
    ...
    cgh.parallel_for<class kernelCompute>( 
      nd_range<1>(range<1>(N), range<1>(wg_size)),
     [=] (nd_item<id> it)
     [[sycl::reqd_work_group_size(1, 1, REQD_WG_SIZE)]] {
       auto gid = it.get_global_id(0);
       accessorRes[gid] = accessorIdx[gid] * 2;
     });